Image-frequency rejection system



Mach 4, 1941.

N. P. CASE IMAGE-FREQUENCY REJECTION SYSTEM Filed Dec. 16, 19259 INVENTOR NELSON P. CASE ATTORNEY Patented Mar. 4, 1941 PATENT OFFICE IMAGE-FREQUENCY REJ ECTION SYSTEM Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela- Application December 16, 1939, Serial No. 309,552 n 8 Claims.

This invention relates to .image-frequency rejection systems and,l more particularly, to carrier-frequency selectors including such rejection systems for modulated-carrier signal superheterodyne receivers.

superheterodyne receivers are subject toan undesirable response which often arises by virtue of the fact that there are signals of two possible frequencies that produce the required intermediate frequency for any given tuning of the receiver. lThe undesired signal has a carrier frequency differing from the carrier frequency of the desired modulated-carrier signal by twice the intermediate frequency and is termed the image-frequency signal or simply the image signal.

'It is known that the image signals can be eliminated b-y preventing them from reaching theY oscillator-modulator stage of the receiver. This requires that the radio-frequency section of the receiver have an adequate number of tuned circuits and that the intermediate frequency -be high enough `to cause such circuits to suppress the image frequencies. In present day multi- F band receivers, an intermediate frequency of about 455 kilocycles is generally used, but imagefrequency suppression is none too good in the high-frequency bands because the frequency difference between the desired and image signals represents only a small percentage of the desired carrier frequency.`

The problem is particularly troublesome in television receivers due to the relatively low intermediate frequencies used therein. This arises because the carrier frequencies are in the ultrahigh-frequency portion of the frequency spectrum, seven television bands having been assigned in the spectrum between 44 megacycles and v103 megacycles. For example, in a television receiver the output of asingle local oscillator is usuallyheterodyned with both sound and picture modulated-carrier signals to produce signals of two intermediate-carrier frequencies of 8.25 megacycles and 12.75 megacycles, respectively. Thus, in receiving the first television channel, assuming a picture modulated-carrier frequency of 45.25 megacycles and a local oscillator frequency of 58 megacycles, the intermediate-carrier frequency is 12.75 megacycles. Thus, the receiver is responsive to an undesired image signal havingv a carrier frequency of '70.75 megacycles, which isin the picture modulated-carrier signal band of the third television channel.

'It is an object ofthe present invention, therefore, to provide vmeans for minimizing spurious (Cl. Z50-20) responses to image-frequency signals in superheterodyne receivers.

It is another object of the invention to provide van improved carrier-frequency signal selector for modulated-carrier signal superheterodyne receivers which is effective to minimize response to image-frequency signals. y

In accordance with the present invention, in

a modulated-carrier signal superheterodyne receiver which is responsive to undesired image- 'frequency signals, there is provided a carrieryelement is coupled thereto whereby image-frequency signals are absorbed by the other fixedinductance element and its associated capacitance and are minimized in the output of the receiver. l

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope Vwill be pointed out in the appended claims.

Figs. 1 and 2 of the drawing are circuit diagrams, partly schematic, of a complete multiband superheterodyne receiver and a complete television superheterodyne receiver, respectively, embodying the invention.

Referring to Fig. 1 of the drawing,l there is shown schematically a superheterodyne receiver including an antenna-ground circuit I0, I0 to which is coupled a tunable radio-frequency selector and amplier unit `I I having an adjustable tuning control unit represented schematically Aby the device I2, the output circuit of the unit Il being coupled to a two-range band-pass selector I3 comprising a pair of fixed-tuned broad band radio-frequency transformers I4 and I5. The primary windings I6 and II of the transformers are connected in series in the output circuit of the unit II, the winding I'I being provided with a short-circuiting switch I 8. The transformer secondaryV windings I9 and 20 are connected for 2 individual connection by a switch 2| to the input circuit of a tunable oscillator-modulator unit 22 and are provided with damping resistors 23 and 24, respectively. The unit 22 is provided with an adjustable tuning control unit represented schematically by the device 25. Control units I2 and 25 are connected for simultaneous operation by a unicontrol mechanism conventionally represented by broken line 25; similarly, band switches I8 and 2| are connected for simultaneous operation by the unicontrol mechanism 21. Coupled in cascade to the oscillator-modulator 22 are an intermediate-frequency amplifier `29 of any desired number of stages, a detector and automatic amplification control or A. V. C.v supk.

ply 39, an audio-frequency. amplifier 3l, and a sound reproducer 32. The A. V. C. supply is connected by way of a lead 33 to the input circuit of oscillator-modulator 22, and t'o the grids of one or more of the vacuum tubes of radio-frequency amplifier II and intermediate-frequency amplifier 29. It will be understood that the several parts of the system illustrated schematically may be conventional in their construction and operation, the details of Whichare well known in the art, rendering description thereof unnecessary herein. Y

Neglecting for the moment the particular operation of the band-pass selector I3 embodying the present invention, the above-described system includes the features of a conventional tworange superheterodyne receiver; the operation of such receiver being well known in the art, a detailed explanation thereof is unnecessary. In brief, however, a desired modulated-carrier signal intercepted by the antenna-ground circuit I0 is selected and amplified in the radio-frequency selector and amplifier unit II, translated by band-pass selector I3 and converted by oscillator-modulator 22 to an intermediate-frequency signal. The intermediate-frequency signal is amplified by intermediate-frequency amplifier 29 and translated therefrom to the detector and A. V. C. supply 3l! wherein the audio frequencies of modulation and automatic Iamplification control potentials are'derived. The audio frequencies of modulation are further amplied in amplifier 3| and delivered to sound reproducer 32 for reproduction in the usual manner. Biasing potentials, developed bythe A. V.. C. supply 39 in a well-understood manner, are applied to the several tubes in the preceding portions of the receiver by way of conductor 33. and are effective to control the amplification in such tubes, thereby to maintain the amplitude of the Ysignal input to the detector within a relatively narrow range or substantially. constant for a wide range of received signal amplitudes. f Y y Referring now more particularly to the parts of the system comprising the present invention, the pass-band of the fixed-tuned transformer I5 may be, for example, the Vbroadcast band which extends from about 540 kilocycles to 1720 kilocycles of the carrier-frequency spectrum and that of .the fixed-tuned transformer I2 may be, for example, the short-wave band which extends from about 2.2 megacycles to 6.7 megacycles of the carrier-frequency spectrum, the responses of the transformers being broadened by the-damping resisto-rs 23 and 24; Thus, the transformers I4 and I5 comprise a plurality of fixed-inductance elements each having capacitance associated therewithand having its constants proportioned to form ja broadly-resonant fixed-tuned circuit. v The reactive, constants.' of -thetransformer I4, that is, its inductances and inherent'.

capacitances, are proportioned to resonate at frequencies in the vicinity of the image frequencies of the wide band of signal frequencies translated by the transformer I5, particularly at signal frequencies in the high-frequency portion of the pass-band of the transformer I5 where image signals are most troublesome. The switches I8 and 2| comprise means for selecting any one of the fixed-inductance elements to translate a wide band of signal lfrequencies.` Thus, the transformer I5 is selected to translate to oscillator-modulator 22 modulated-carrier signals in the broadcast frequency band by operating they switch I8 to open the short circuit across Athe primary winding I'I of the transformery I5 and operating switch 2I to connect oscillatormodulatork 22 to the high potential. terminal of secondary winding 20. By virtue of this conf nection, the windings of transformer I4 are coupled, inductively through proximity of the coil forms,A or capacitively through the switch and wiring capacitances, or by a combination of both, to the corresponding windings of transformer vI5 and are tuned by their inherent capacitances to act as traps for,r or absorb, imagefrequency signals of, desired signals, especially those of the higher-frequency signals in the broadcast band. Asv a resultimage signals in the output of the receiver are minimized. v

In operation, the unicontrol mechanism 21 is rst moved to select a desired band of frequencies vand the uni-control mechanism 26 is then moved to tune the radio-frequency selector II and the oscillator-modulator 22 to-v the carrier frequency of any desired modulated-carrier signal Within the selected frequencyV band. The

band-pass selector I3 automaticallyV minimizesl response of the receiver to image-frequency signals when operating in the low-.frequency band,v particularly the higher-frequency portion thereof. v y l Referring to Fig. 2 of the drawing,.there is shown a signal selector in accordance with the invention utilized as the input selector in a modulated-carrier television superheterodyne receiver. The receiver includes a doublet antenna 40 coupled by a selector 4I, constructed in accordance with'the invention, tothe tunable oscillatormodulator 42 Yof the receiver. Neglecting-for'the moment the parts of the selector circuit V4I constituting the present invention, vthe receiver comprises the following additional elements coupled'incascade, in the order named an inter'- mediate-frequency amplifier 43, a detector 44, aA

video-frequency amplifier 45, and an image-rej producing device 46, Whichvmay be a 'cathoderay signal-reproducing)k tube. It Will be understood that suitablescanning apparatus for rthevv cept selectorM, may a-ll be of conventional -Wellknown constructionrso ythat detailed illustrations and descriptions thereof are deemed to be url-n necessary.

Referring briefly, however, to vthe A.iperation of the system described above, television signals in.-k

tercepted by antenna 4:0` are' selected by selector ,4l and supplied to oscillator-.modulator42-wherein they vare converted to, intermediate-frequency signals which,in turn, are :selectively amplified in intermediate-'frequency amplifier llt. and delivered todetector 44. The. modulation .compo- .nents of the 'signalV are derived y'by-the, detectorV ydevice 46 in the usual manner.

44 and supplied to the video-frequency amplier 45 wherein they are amplified and from which they are supplied in the usual manner to a brilliancy-control element of the image-reproducing device 46. The intensity .of the scanning beam of device 46 is thus modulated or controlled in accordance with the light-modulation voltages impressed on the control element of the Synchronized scanning fields are generated and applied to the device 46, thereby to deflect the scanning ray in directions normal to each other so as to trace a rectilinear scanning pattern on the screen of the tube and reconstruct the transmitted image.

Referring now more particularly 'to the selector system 4| of Fig. 2 embodying the present vinvention, there is shown a fixed-inductance transformer comprising primary windings 41, 48, cou- -pled by condensers 49, 50 and atransmission line v5| to antenna40, and a secondary winding 52.

Inductance elements 53, 54, and of progre..- sively decreasing inductance areadapted to be selectively coupled in parallel with Winding 52 in the input circuit of the oscillator-modulator 42 by a station-selector switch 56 .which is operated with a station-selector switch 51 of the oscillatormodulator unit 42 by a unicontrolmechanism 53, shown conventionally by broken lines. The winding 52 and each of the inductances 53-55, inclusive, is effective to tune the input circuit of vunit 42 to predetermined modulated-carrier signals of progressively increasing carrier frequencies. Furthermore, each of the inductance elements E53- 55, inclusive, has its reactive constants, including its distributed capacitance and .the distributed capacitance of its associated circuit and switchcontact, proportioned to resonate at a frequency in the vicinity of the image frequency of the modulated-carrier signals designed to be translated by the'circuit including the adjacent inductance element of next higher inductance. The lumped capacitances to ground of the inductances 53-55, inclusive, respectively, are represented by condenser 62 shown in dotted lines. The capacitances 64, 65 between the switch contacts and leads are also represented by dotted lines, as arethe capacitances 61,' 68, 59 between theA switch contacts and ground.

In considering the operationA of the signal selector 4i, the unicontrol mechanism 58, preferably a push-button system, is adjusted to select desired modulated-carrier signals on a particular carrier frequency, for example, the lowest carrier frequency for which the selector 4I is designed. Thus, the switch 56 is moved to position 10 to couple the transformer winding 52 alone in the input circuit of the oscillator-modulator 42, which -is'simultaneously adjusted by the switch 5'! to the proper oscillator frequency. The Winding 52 thereupon supplies modulated-carrier signals of the desired frequency to the unit 42. At the same time the adjacent fixed-tuned circuit, including inductance 53, resonates at a frequency in the vicinity of the image frequency of the desired modulated-carrier signals and, being coupled thereto by capacitance 61, acts to trap or absorb image-frequency signals and thus suppress them from the succeeding portions of the receiver.

While there have been described what are at present considered to be the ,preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing vfrom the invention, and it is, therefore, aimed in the appended'claims to' cover 'all' such changes and modifications as fall'within thetrue spirit and scope of the invention'.

What is claimed is: 1.v In a modulated-carrier signal superheterodyne receiver responsive to undesired image-freance elements and its associated capacitance having reactive constants proportioned to resonate at frequencies in the Vicinity of the image frequencies of said wide band of signalfrequencies, said other fixed-inductance element, upon selection of -said one fixed-inductance element, being coupled thereto, whereby image-frequency signals are absorbed by said other fixed-inductance `element and its associated capacitance.

2. In a modulated-carrier signal superheterodyne vreceiver responsive to undesired image-frequency signals, a carrier-frequency signal selector comprising, a plurality of Xed-inductance 'elementsv each having capacitance associated therewith and having its constants proportioned to form a broadly-resonant Xed-tuned circuit to translate a respective one of a plurality of vwide bandsfof frequencies, and means for selecting any one of said fixed-inductance elements to translate a predetermined wide band of signal frequencies, another of said fixed-inductance elements and its associated capacitance having reactive constants proportioned to resonate at frequencies in the vicinity of the image frequency of said predetermined wide band of signal frequencies, said other iiXed-inductance element, upon selection of said one fixed-inductance element, being coupled thereto, whereby imagefrequency signals are absorbed by said other fixed-inductance element and its associated capacitance.

3. In a modulated-carrier signal superheterodyne receiver responsive to undesired image-frequency signals, a carrier-frequency signal selector comprising, a plurality of progressively decreasing Xed-inductance elements each having capacitance associated therewith and having its constants proportioned to form a broadly-resonant fixed-tuned circuit to translate a respective one of a. plurality of wide bands of signal frequencies, land'means for selecting any one of said fixed-inductance elements to translate a predetermined wide band of signal frequencies, an adjacent fixed-inductance element of lower inductance audits associated capacitance having reactive constantsv proportioned to resonate at frequencies in the vicinity of the image frequency of said predetermined wide band of signal frequencies, said adjacent fixed-inductance element, upon selection of said one fixed-inductance element, being coupled thereto, whereby imagefrequency signals are absorbed by said adjacent fixed-inductance element and its associated capacitance.

4. In a modulated-carrier signal superheterodyne receiver responsive to undesired image-frequency signals, a carrier-frequency signal selector comprising, a plurality of iixed-inductance elements each having capacitance associated therewith and having its constants proportioned to form a broadly-resonant fixed-tuned circuit of progressively increasing inductance and distributed capacitance and each tuned to a different predetermined signal-frequency band, means for selecting any one of said fixed-inductance elements to translate a predetermined wide band of signal frequencies, the fixed-inductance element of lower inductance and its associated capacitance having reactive constants proportioned to resonate at frequencies in the vicinity of the image frequency of said predetermined wide band of signal frequencies, said Xed-inductance element of lower inductance, upon selection of said one f'lXed-inductance element, being coupled thereto, whereby imagefrequency signals are absorbed by said xed-inductance element of lower inductance and its associated capacitance.

5. In a multiband modulated-carrier signal superheterodyne receiver responsive to undesired image-frequency signals, a multiband-pass selector comprising, a plurality of fixed-tuned inductance elements each having capacitance associated therewith and having its constants proportioned to form a broadly-resonant fixed-tuned circuit tuned to pass modulated-carrier signals in a predetermined broad frequency band, means for selecting any one of said iiXed-inductance elements to translate desired modulated-carrier signals in a broad frequency band, another of said fixed-inductance elements and its associated capacitance having reactive constants proportioned to resonate at frequencies in thevicinity of the image frequency of the desired modulatedcarrier signals, said other xed-inductance element, upon selection of said one Xed-inductance element, being coupled thereto, whereby imagefrequency signals are absorbed by said other iixed-inductance element and its associated capacitance.

6. In a multiband modulated-carrier signal superheterodyne receiver responsive to. undesired image-frequency signals, a multiband-pass selector compri-sing, a plurality of fixed-tuned pairs of inductively-coupled fixed-inductance elements each pair having capacitance associated therewith and having its constants proportioned to form broadly-resonant fixed-tuned circuits tuned to pass modulated-carrier signals in a predetermined frequency band, means for selecting one pair of said inductively-coupled xed-inductance elements to translate desired modulated-carrier signals in a broad frequency band, another pai-r of ,saidv inductively-coupled xed-inductance ele- Ine-nts and their associated capacitance having reactive constants proportioned to resonate at frequencies. in the vicinity of the image-frequency of the desired modulated-carrier signals, said other pair of inductively-coupled fixed-inductance. elements, uponvselection of said one pair of inductively-coupled iixed-inductance elements,

being coupled thereto, whereby image-frequency signals are absorbed by said other pairv of in- Vductively coupled xed-inductance elements and their associated capacitance.

7. In a multiband modulated-carrier signal superheterodyne receiver responsive to undesired image-frequency signals, a multiband-pass selector comprising, a plurality of broad band radiofrequency transformers of xed-inductance having capacitance eachfassociated therewith and hayingitsconstants proportioned to form broad- -ly-resonant fixed-tunedl circuits tuned to pass modulated-carrier signals in a predetermined separate frequency band, means for selecting one y of said transformers to translate desired modulated-carrier signals in a broad frequency band, another of said transformers and its associated capacitance having reactive constants proportioned to resonate at frequencies in the vicinity of the imagefrequency of the desired modulatedcarrier signals, said other transformer, upon selection of said one transformer, being coupled thereto,l whereby image-frequency signals are absorbed by said other transformer and its associated capacitance.

8. In a multiband modulated-carrier signal superheterodyne receiver responsive to undesired image-frequency signals and including a radio.- frequency vacuum-tube amplifier and a tunable oscillator-modulator stage, a two-band band-pass selector comprising, a first broad band radiofrequency transformer of fixed-inductance having capacitance associated therewith and adapted to form a fixed-tunedA circuit tuned to pass modulated-carrier signals in the shortwave frequency band, a second broad band radio-frequency transformer of flXed-induct-ance having capacitance associated therewith and adapted to form a fixedtuned circuit tuned to pass' modulated-carrierA resonate at frequencies in the image-frequency band of the` broadcast frequency band, said first transformer, upon vselection of said second transformer, being coupled thereto, whereby imagefrequency signals are absorbed by said other transformer andv its associated capacitance.

NELSON P. CASE. 

